The present invention relates generally to systems for recovering information from a record medium, and, more particularly, to systems for recovering information from an information storage disc that is rotatable at a variable angular velocity.
Information storage discs having a plurality of substantially circular and concentrically arranged information tracks are becoming increasingly popular, particularly when used for storing video signals. Each information track on the disc typically stores one complete frame of the video signal, so recovery of the video signal at a prescribed constant rate necessary for proper reception of the signal by a conventional television receiver ordinarily necessitates a rotation of the disc relative to a transducer at a substantially constant angular velocity.
An example of a servo apparatus for recovering a video signal from such a disc is described in a co-pending and commonly assigned application for U.S. patent Ser. No. 890,670, filed in the name of W. R. Dakin et al and entitled "Video Disc Player". The servo apparatus described in the application includes a spindle motor for rotating the disc, a tachometer coupled to the motor for producing a signal having a frequency proportional to its angular velocity, and a phase detector for comparing the phase angle of the tachometer signal with that of a periodic reference signal. The output of the phase detector, which is proportional to the phase difference of its two input signals, is suitably amplified and coupled to the motor to rotate the disc at the appropriate constant angular velocity.
An example of another servo apparatus for recovering a video signal from such a disc is described in a co-pending and commonly assigned application for U.S. patent, Ser. No. 920,701, filed in the name of J. S. Winslow et al and entitled System for Recovering Information from a Movable Information Storage Medium. The servo apparatus described in the application is similar to the apparatus described previously, except that substituted for the tachometer signal is a pilot signal, which has been extracted from the signal recovered from the disc. This system operates more effectively to recover the signal from the disc at a prescribed constant rate, however, since it is not as susceptible to errors caused by relative slippage of the disc and eccentricities of the disc.
Since information is recovered from a disc of the type described above by rotating it at a constant angular velocity, and since each information track extends over a full circumference on the disc, a video signal stored in tracks at the periphery of the disc is less densely recorded than a signal stored nearer the center of the disc. The entire information recording capability of the disc is thus not utilized to its fullest extent, and, as a result, the playing time of the disc is substantially lower than it otherwise could be.
One technique for increasing the playing time of an information storage disc is disclosed in a co-pending and commonly assigned application for U.S. patent Ser. No. 961,405, filed simultaneously herewith by the same inventors and entitled System for Recording Information on a Rotatable Storage Disc, In a Substantially Uniform Recording Density. In that application, a technique is described for recording information onto a storage disc of the aforedescribed type, wherein the the disc is rotated relative to a radially movable transducer at an angular velocity that is varied according to the radius of the particular information track in question. In particular, a substantially uniform recording density over the entire disc can be achieved by rotating the disc at an angular velocity inversely proportional to the radius of the selected track. The amount of information recorded on each information track is thus generally proportional to the length of the track. Since known prior apparatus for recovering information from rotatable information storage discs operate to rotate the disc only at a constant angular velocity relative to a transducer, they are generally incapable of recovering information from discs of the type having such a uniform recording density.
It will thus be appreciated that there is a need for a method and apparatus for rotating an information storage disc of the type described above at a variable angular velocity relative to a transducer, to recover information from the disc at a prescribed constant rate. Moreover, there is a particular need or a method and apparatus for recovering information from a disc having a uniform recording density. The present invention fulfills these needs.